1. Field of the Invention
The present invention relates to a developer for developing latent electrostatic images to visible images in a developing process in the fields of electrophotography, electrostatic recording and electrostatic printing; and a method of forming images by using the developer.
2. Discussion of Background
According to the electrophotographic image formation method based on the Carlson process, which is now widely employed, image formation is basically carried out in such a manner that the surface of a photoconductor is uniformly charged to a predetermined polarity and the photoconductor thus charged is selectively exposed to the original light images to form latent electrostatic images on the photoconductor. Then, the latent electrostatic images are developed with a developer, so that visible toner images can be obtained on the photoconductor. The visible toner images are then transferred to a sheet of an image-receiving medium and fixed thereon.
On the other hand, many proposals on the image formation method not using the Carlson process, but using the rear side exposure system have been reported, for example, in The Journal of the Institute of Image Electronics Engineers of Japan vol. 16, (5), 306 (1987); and Japanese Laid-Open Patent Applications 61-149968, 63-10071 and 63-214781, by which rear side exposure system the image formation apparatus can be made compact and the image formation process can be made simple.
In the rear side exposure system, the surface of the photoconductor is provided with a developer to form a developer resident portion, through which the photoconductor is subjected to a cleaning operation, and the photoconductor is uniformly charged. The light images are applied to the photoconductor from the rear side thereof and the latent images formed on the surface of the photoconductor are simultaneously developed into toner images with the developer.
However, there are too many difficult problems in the rear side exposure system to put it into practice. More specifically, the requirements for each function in the rear side exposure system are made extremely severe because it is necessary to inject the electric charge sufficient for the development into the photoconductor through the developer accumulated in the developer resident portion and to form sharp and stable toner images on the photoconductor by development at a relatively small developer resident portion.
In addition, it is necessary to impart the electroconductivity to a developer since the electric charge is injected into the photoconductor through the developer. Therefore, when a developer to be employed is a one-component type developer, an electroconductive magnetic toner is essentially required. The toner image thus formed on the photoconductor cannot be transferred to a sheet of plain paper by the electrostatic image transfer method such as corona transfer or bias roller transfer. As a result, only a sheet of paper with high resistivity can be used in this system.
The method of forming a multi-colored image on a sheet of plain paper by the rear side exposure system is disclosed in Japanese Patent Publication 60-59592. In this method, however, since a photoconductor is prepared by overlaying an insulating layer on a photoconductive layer, the photoconductor cannot stand the repetition of formation of multi-colored images thereon. To solve this problem, it is proposed that the residual latent image formed on the photoconductor be erased by application thereto of a transfer electrical field. This proposal is still insufficient in practice for obtaining clear images over an extended period of time.
As in the Journal of the Institute of Electrophotography Engineers of Japan vol. 27, No. 3, p.442 (1988) and Japanese Laid-Open Patent Application 61-46961, the image formation can be achieved by the rear side exposure and the simultaneous development system, with the application of a charging bias and a development bias to a photoconductor, having counter polarities, using a two-component type developer comprising iron carrier particles with a resistivity of 10.sup.4 to 10.sup.8 .OMEGA..cm and magnetic toner particles with electrically insulating properties.
However, when the above-mentioned image formation method is applied to the practically-used copying apparatus, it is difficult to control the image formation system for obtaining a clear image over an extended period of time, and in addition, the structure of the apparatus necessarily becomes complicated.
Furthermore, there are disclosed a variety of image forming methods by use of a developer comprising a magnetic carrier prepared by dispersing a magnetic material in a binder resin. For example, a developer comprising the above-mentioned magnetic carrier and an electrically insulating non-magnetic toner is proposed in Japanese Laid-Open Patent Applications 53-33152 and 55-41450; and a developer comprising the above-mentioned magnetic carrier and an electrically insulating magnetic toner is proposed in Japanese Laid-Open Patent Applications 53-33152, 53-33633 and 53-35546. In these disclosures, the carrier component in a developer has insulating properties and the development is carried out by the conventional Carlson process.
In a two-component developer as disclosed in Japanese Laid-Open Patent Application 57-204570, two kinds of magnetic carriers are used in combination, with one magnetic carrier having higher electroconductivity and larger particle diameter as compared with the other magnetic carrier. Using such a two-component developer, development is carried out with a development bias voltage and a pulse voltage applied to a development sleeve. This image forming method is not based on the rear side exposure system, but the Carlson process.
The applicants of the present application have proposed an electroconductive magnetic resin carrier suitable for the rear side exposure system, which is prepared by forming an electroconductive layer on the surface of a base particle comprising a binder resin and a magnetic material dispersed in the binder resin, and an image forming method based on the rear side exposure system using the above-mentioned carrier, as disclosed in Japanese Laid-Open Patent Application 5-80591.
When a two-component developer comprising the above-mentioned electroconductive magnetic resin carrier and an electrically insulating toner is used to carry out the image formation on the basis of the rear side exposure system. In the image forming procedure by this method, a development bias voltage is applied to a development drum and electric charges are thus injected into a photoconductor through the electroconductive magnetic resin carrier, thereby charging the photoconductor to a predetermined polarity. In order to provide the photoconductor with the required charge quantity and carry out the image formation in a stable condition, therefore, it is necessary to decrease and stabilize the resistivity of the developer.
However, the resistivity of the above-mentioned electroconductive magnetic resin carrier is not always sufficiently low, and the resistivity of a developer comprising this type of electroconductive magnetic resin carrier is apt to increase due to deterioration of the developer during repeated operations for a long period of time. As a result, the surface of the photoconductor cannot be uniformly charged.
Furthermore, a coated-type carrier which is prepared by coating a base particle with a polyolefin resin is disclosed, for example, in Japanese Laid-Open Patent Applications 2-187770, 2-187771, 3-208060 and 4-70853. In these applications, the following descriptions are given:
(1) The synthetic resin layer can be formed on a base particle by polymerizing monomers directly on the surface of the base particle in accordance with the method described in Japanese Laid-Open Patent Application 60-106808. PA1 (2) The surface of the resin-coated-type carrier particle thus obtained can be provided with convex and concave portions, with a shape factor of 130 to 200. PA1 (3) The surface profile of the resin-coated-type carrier particles can be controlled by heat treatment after the formation of the synthetic resin layer on each base particle. PA1 (4) The base particle for this resin-coated-type carrier may essentially consist of magnetic powder such as iron, ferrite or magnetite, or comprise a binder resin and finely-divided particles of a magnetic material dispersed in the binder resin. PA1 (5) The synthetic resin layer provided on the base particle may further comprise finely-divided particles of an electroconductive material such as carbon black.
According to the aforementioned applications, the resistivity of the synthetic resin layer formed on the base carrier particle is preferably in the range of 1.times.10.sup.6 to 1.times.10.sup.14 .OMEGA..cm, more preferably in the range of 10.sup.8 to 10.sup.13 .OMEGA..cm, and further preferably in the range of 10.sup.9 to 10.sup.12 .OMEGA..cm. Further, it is said that the resistivity of the resin-coated-type carrier can appropriately be decreased by the addition of electroconductive finely-divided particles such as carbon black to the synthetic resin layer, and consequently an adequate balance is maintained between leakage of electric charges from the photoconductor and accumulation of electric charges thereon, and therefore, the development performance can be improved and images can be obtained with high image density and clear contrast. It is obvious from the above descriptions that this kind of coated-type carrier is oriented to an electrically insulating carrier for charging a toner, and it is not suggested that this coated-type carrier be used as an electroconductive carrier. In addition, image formation is carried out using commercially available copying machine based on the Carlson process in all of the above-mentioned applications, and there is no suggestion that the image formation be carried out on the basis of the rear side exposure system using this resin-coated-type carrier.